Memory devices using phase change materials (PCMs) have become an active research and development issue. Since the structural phase changes of the crystalline and amorphous phases occur at a high temperature in PCMs, PCMs may be used for memory devices. However, PCMs are not used in other fields, for example, for switching devices. This is because the fast switching speed cannot be achieved by PCMs due to a position change of atom caused by the structural phase change.
Mott-Hubbard field effect transistors (FETs) use Mott-Hubbard insulators as channel layers and were suggested as an example of switching devices using phase changes by D. M. Newns et al. [Appl. Phys. Lett. 73 (1998) 780]. Mott-Hubbard FETs are turned on and/or off according to a metal-insulator transition and do not include depletion regions, unlike general MOSFETs. Thus, the Mott-Hubbard FETs show a higher speed switching characteristic than the general MOSFETs, and integration thereof can be greatly improved. However, since the Mott-Hubbard FETs use a continuous metal-insulator transition, the carrier electric charge must be continuously added until the Mott-Hubbard FETs show the best metallic characteristics. Thus, the added electric charge will add up to a high density. As a result, either a dielectric constant of the gate insulating layer in a Mott-Hubbard FET must be large, or a thickness of the gate insulating layer must be thin, or a gate voltage applied must be large. However, if the dielectric constant is too large, the dielectric is sharply deteriorated during a high switching operation, and this will lead to shortening of the transistor lifespan. It is difficult to make the thickness of the gate insulating layer thin due to process limitations. If the gate voltage is increased, the power consumption is also increased, and this will cause the transistor unsuitable for low power applications.
U.S. Pat. No. 6,624,463 discloses an abrupt metal-insulator transition device using an abrupt metal-insulator transition material in an attempt to solve the above-described problems. The abrupt metal-insulator transition material adds low density holes to a Mott-Brinkman-Reiss insulator so that a transition from an insulator state to a metal state occurs not continuously but abruptly. The hole-driven metal-insulator transition theory was suggested by Hyun-Tak Kim in the paper ‘New Trends in Superconductivity’ [NATO Science Series Vol II/67 (Kluwer, 2002) p137]’ and http://xxx.lanl.gow/abs/cond-mat/0110112. Since the density of the added holes is very low, the problems of FETs using metal-insulator transition materials can be solved.
However, when a metal-insulator phase change occurs in an abrupt metal-insulator transition device, a great current flows abruptly between source and drain electrodes and thus a high temperature exothermic operation occurs.
FIG. 1 is a graph illustrating current (I)-voltage (V) characteristics of an abrupt metal-insulator transition device. Referring to FIG. 1, a great drain current flows abruptly at a drain voltage of about 27V. When a gap between source and drain electrodes is about 5 μm, and a line width of a gate electrode is 25 μcm, a density of a current flowing between the source and drain electrodes is about 5×10E5 A/cm2. Thus, a very large current flows between the source and drain electrodes, and the device is heated via Joule heating.
In a current abrupt metal-insulator transition device, Cr/Au is used as the source and drain electrodes. However, Cr/Au cannot sustain a high current density and is thermally degraded. Thus, as shown in FIG. 2, the source and drain electrodes above a channel are broken down. As a result, the characteristic of the abrupt metal-insulator transition device is deteriorated. In a case of serious deterioration, the abrupt metal-insulator transition device cannot be used any more.